Massively Multiplayer Online Game

Massively Multiplayer Online Game, often abbreviated to just MMO rather than the full MMOG refers to games that are played by thousands of players simultaneously.

Common usage

The most common usage is when you talk about RPGs played online in large scales, MMORPGs, but really refers to any game played with a large player base.

Diablo

It's fairly easy to say that Diablo II and Diablo III are both MMOs, even if not MMORPGs. However, the genre is mostly referred to as ARPG, lookign at gameplay rather than player base.